Mechanical vibration testers of various kinds are in common use today for generating low frequency sinusoidal and steady-state vibrations. Such mechanical shakers, which are capable of operating in the approximate range of 0.1 to 100 Hz, vibrate at a single test frequency at all times. The test frequency may be held constant, or it may be varied to subject the work piece to vibrations throughout a range of different frequencies.
In the past, it has proven difficult and expensive to provide mechanical vibration test equipment which is capable of operating very much above the 100 Hz range, e.g. at frequencies ranging up to 3,000 Hz which are called for in certain tests.
From experience it has been found that single-frequency testing of work pieces, while satisfactory for certain purposes, simulates only a limited set of conditions. Under actual operating conditions random or complex vibration is induced in many instances. Such vibration results in far more stresses and strains in the equipment subjected thereto than are produced where only simple vibration is present. Accordingly, random or complex vibration testing of work pieces is necessary since it induces far more failures in inherently weak components or structures than show up in more conventional single-frequency sinusoidal tests. Conversely, work pieces that have been subjected to extensive periods of random or complex vibration testing and which have exhibited an acceptably low failure rate, or wherein the weak points have been exposed and corrected, have been shown to perform reliably under actual operating conditions. As used herein, the term complex vibration refers to an approximation of random vibration in a predetermined frequency spectrum using a limited number of discrete frequencies.
Past attempts to use mechanical vibrators to impart random or complex vibration to a work piece, i.e. to subject the work piece simultaneously to a number of discrete frequencies, have largely been unsuccessful. Thus, the vibration levels applicable to random or complex vibration testing are commonly specified in power spectral density units (G.sup.2 /Hz) as a function of frequency. See for example Military Standard MIL-STD-810C, entitled "Environmental Test Methods", dated Mar. 10, 1975. Note particularly Section 514.2, FIGS. 514.2-2A, 514.2-4, 514.2-4A and 514.2-5, among others. In the past, it has proven difficult to provide simple, inexpensive mechanical vibration test equipment in which vibration levels so specified could be readily furnished.
Presently available equipment for performing random or complex vibration testing generally takes the form of electrodynamic test apparatus. In such equipment the vibrations are electrodynamically imparted to one or more diaphragms, analogous to the manner in which vibrations are produced in a loudspeaker. In addition to being costly, equipment of this type is by necessity sophisticated and frequently requires extensive maintenance to keep it operating properly over long periods of time. Further, the complexity of the required maintenance and repair operations requires skills that are often beyond the scope of the user's personnel.